Nucleation of solidification in liquid droplets
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I.
INTRODUCTION
T H E nucleation of solidification in an ingot or casting determines many important aspects of the resulting microstructure, such as the grain size, grain shape, segregation pattern, and distibution of second-phase particles, aLl of which have a significant influence on the final material properties. Unfortunately, controlled and reproducible experimental techniques for investigating the nucleation of solidification are difficult to achieve, because the nucleation process is highly sensitive to the presence of catalytic trace impurities. The main method which has been employed to try to overcome this problem has been to divide the mass of liquid into a large number of small droplets in order to segregate any catalytic trace impurities into relatively few droplets and render them harmless. [1-2s] Calorimetry, dilatometry, or optical microscopy has then been used to monitor the solidification behavior of the droplets when they are cooled below their equilibrium melting point. When an organic fluid is used to manufacture a liquid droplet foam for nucleation experiments of this type, the typical droplet size is 10 /.tm and the resulting droplet solidification behavior is still not very reproducible. Ct-'6] As shown in Table I, measurements of undercooling in the liquid before the onset of solidification are found to vary by as much as -+50 K, implying that the desired segregation of catalytic trace impurities has not been fully achieved. When conventional casting techniques are used to manufacture a dispersion of approximately 10/~m sized liquid droplets embedded in a solid matrix, the repro-
B. CANTOR, Director, and D.L. ZHANG and W.T. KIM, Postdoctoral Research Assistants, are with the Oxford Centre for Advanced Materials and Composites, Department of Materials, Oxford University, Oxford OXI 3PH, England. Manuscript submitted April 3, 1990. METALLURGICAL TRANSACTIONS A
ducibility of the liquid undercooling experiments is improved considerably, to -+5 K . [17-2~ However, the best results are obtained when rapid solidification is used to manufacture a dispersion of much smaller, approximately 10 nm sized, liquid droplets embedded in a solid matrix, with liquid undercooling measurements then reproducible to better than -+0.5 K. t2~-28]With 10 nm sized liquid droplets embedded in a solid matrix, transmission electron microscopy can be used to examine in detail the solidified droplet microstructures and crystallography; and with +0.5 K reproducibility in liquid undercooling measurements, differential scanning calorimetry can be used to examine in detail the droplet solidification kinetics as a function of the cooling rate in the calorimeter, t2~-2s] Figures l(a) through (c) show a typical series of differential scanning calorimeter results taken from the work of Zhang et al. [22]on a rapidly solidified hypomonotectic alloy of A1-4.5 wt pct Cd. Specimens of the as-rapidly solidified alloy were heated in the calorimeter up to a temperature of 350 ~ i . e . , 29 K above the equilibrium Cd melting point of
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